Field of the Invention
The present invention relates to an image forming apparatus and an optical scanning apparatus, such as a laser beam printer, a copy machine or a fax machine, that form an image by scanning light.
Description of the Related Art
There are image forming apparatuses that form an image by exposing a photosensitive member. Furthermore, some of these image forming apparatuses form a light spot on the surface of the photosensitive member by reflecting light with a rotating polygon mirror and focusing the reflected light using a scanning lens. By rotating the rotating polygon mirror, the light spot moves over the surface of the photosensitive member in a main scanning direction (direction orthogonal to a circumferential direction of the photosensitive member), and thereby forms a latent image on the photosensitive member.
Note that lenses having fθ characteristics are mainly used as the scanning lens. This is to ensure that the light spot moves at a uniform speed over the surface of the photosensitive member, when the rotating polygon mirror rotates at a uniform angular velocity. However, scanning lenses having fθ characteristics are comparatively large and costly. Thus, configurations that do not using a scanning lens or that use a scanning lens that does not have fθ characteristics are being considered with the aim of reducing the size and cost of image forming apparatuses. Japanese Patent Laid-Open No. 58-125064 discloses a configuration that changes the clock frequency during the scanning of one scan line, such that dots that are formed on the surface of the photosensitive member have a constant width, even when the light spot does not move over the surface of the photosensitive member at a uniform speed.
Image forming apparatuses are required to perform exposure that suppresses image distortion by making a LSF (Line Spread Function) profile of each pixel (dot) uniform in the main scanning direction. This still applies even when not using a scanning lens having fθ characteristics.